THE DISCOVERY OF 8.7 2ND PULSATIONS FROM THE ULTRASOFT X-RAY SOURCE 4U-0142+61

We discovered an X-ray periodicity at approximately 8.7 s from the dir ection of the two sources 4U 0142+61 and RX J0146.9+6121. The pulsatio ns are visible only in the 1-4 keV range, during an observation obtain ed with EXOSAT in 1984 August. In the same data, periodic oscillations at 25 minutes had been previously found in an additional hard spectra l component above 4 keV. The newly discovered periodicity most likely originates from the optically unidentified source 4U 0142+61, previous ly considered a possible black hole candidate on the basis of its ultr asoft spectrum. Marginal evidence for the approximately 8.7 s pulsatio ns is found in the two 1985 EXOSAT observations and in a 1991 ROSAT hi gh-resolution imager pointing; if true, these measurements imply a spi n-up timescale of approximately 530 yr. Although the very high (> 10(4 )) X-ray to optical flux ratio of 4U 0142+61 is compatible with models based on an isolated neutron star, the simplest explanation involves a low-mass X-ray binary with a faint companion, similar to 4U 1626-67. A search for delays in the pulse arrival times caused by orbital moti on gave negative results. The discovery of periodic pulsations from 4U 0142+61 weakens the phenomenological criterion that an ultrasoft spec tral component is a signature of accreting black holes.